With all the new projects coming to completion in combination with spec tax and other factors I am seeing a lot of brand-new re-sale units coming to market, with advertising along the lines of "No GST, Already Paid." If a unit has been left unoccupied, it is still considered new upon resale of the unit. The federal government considers this transaction a transfer of unoccupied inventory so GST would be payable upon the resale transaction.
As a seller super important to consult your accountant on this matter and as a buyer SUPER important your REALTOR® drafts the contract correctly so you don't end up with a GST bill.
Don't fall for the trap of "seller paid the GST two weeks ago when he or she completed on the unit, so it can't possibly by payable again."